1. Field of the Invention
The present invention relates to a battery charger. More particularly, it relates to a battery charge equalizer for a series of connected battery strings to be equally charged.
2. Description of the Prior Art
Charge equalization enhances the uniformity of the battery. Difference in battery chemistry during repeated cycles of battery charge and discharge, lead to large nonuniformities in battery charge levels and corresponding differences in battery terminal voltages. During charging of a battery composed of a series of batteries, some batteries will consequently reach full charge before others and before the overall battery terminal voltage reaches its nominal value. Such a process leads to overcharging of a subset of batteries. If these batteries are charged into the gassing phase, there can be significant degradation of the battery life. If battery strings monitoring and recharging is done on a single-battery basis, it is possible to maintain each battery at its optimal operating point, and to maximize battery life. Hence, it is usual to use a power converter to provide charge equalization for all the batteries in a series string.
Referring to FIG. 1, there is a conventional battery charge equalizer constituted with shunt resistors, wherein B1, B2, B3 and B4 represent a series of connected battery strings; R1, R2, R3 and R4 represent resistors; SW1, SW2, SW3 and SW4 represent switches; and 1 represents a battery voltage sensing and controlling circuits. The battery voltage sensing and controlling circuits 1 detects the charging condition of each battery of the series of connected battery strings. As any one of the batteries, such as battery B1 has reached its pre-determined voltage value earlier than others and is detected by the battery voltage sensing and controlling circuits 1, the battery voltage sensing and controlling circuits 1 outputs an OP1 signal to turn on the switch SW1 letting a part of current which originally charges into battery B1 pass through the resistor R1 to maintain the battery B1 voltage without overcharging it, so as to avoid damaging the battery itself Hence the series of connected battery strings B1, B2, B3 and B4 can be charged to a proper voltage level without overcharging. But this type of battery charge equalizer with shunt resistors will waste energy and is inefficient.
There is another type of conventional battery charge equalizer, which is constructed with a flyback converter with a transformer consisting of one primary coil and a number of identical secondary coils equal to the number of batteries, which draws out current from the whole battery system and directs the output energy to the batteries with the lowest voltage. An example of battery charge equalizer with a flyback converter consisting of three series of connected batteries B1, B2, B3 is shown in FIG. 2. Under ideal conditions, this system is in the standby mode. A simple comparative circuit (not shown in FIG. 2) monitors the battery voltages. The flyback converter is activated as soon as one of the voltages deviates by more than a pre-determined tolerance value. A switch SW5 will be switched on, and off with high frequency and energy will be transferred from the whole battery via the transformer and the rectifying diodes D11, D12, D13 on the battery B1, B2, B3 respectively to the individual battery again. During this process, the battery with the lowest voltage will determine the voltages induced in the secondary coils. As all the coils are mounted on a common core, all the voltages induced in the secondary coils are equal, and the largest proportion of the secondary current will flow into the battery with the lowest voltage without the need for any additional selection logic, so that the charge of series of connected battery strings B1, B2, B3 and B4 can be maintained to a proper voltage without overcharging.
There are two difficulties in this type of battery charge equalizer constructed with a flyback converter. One of them is that the battery charge equalizer constructed with a flyback converter requires a plurality of sets of secondary coils charged equally in a series of connected battery strings with a plurality of batteries, which mounts all the coils on a common core to render all the voltages induced in the secondary coils equal. Thus the structure of transformer is complicated and is difficult to manufacture. The other difficulty is that the battery charge equalizer constructed with a flyback converter, the transformer of the flyback converter is not easy to symmetrize as each mounted coil on a common core has a plurality of sets in the secondary coil, so the effect of charge equalization is reduced. Furthermore, the type of battery charge equalizer constructed with a flyback converter in the various numbers of the batteries of the series of the connected battery strings is not conveniently adjustable.